The BBeB viewer on the Reader is set up for full justification for all non-japanese books. There is a possibility to disable justification, but it's not exactly configurable. To do that, add the following line to the <environment> section of kconfig.xml:

The most you can do is set justification to right or center. Left/full can be toggles only globally.
BBeB was originally developed for Japanese market and many details irrelevant for Japanese seem to have been overlooked. Sometimes I think it's pretty remarkable it works well enough for Western books...